A communication system is sometimes conceptually described by an Open Systems Interconnection model (OSI). The model partitions internal functions of the communication system into abstraction layers, such as physical layer (layer 1), data link layer (layer 2), network layer (layer 3), transport layer (layer 4), etc.
An Ethernet network, as an example of a known communication system, is often considered to be part of layer 2. The Ethernet network is also an example of a so-called switched network, which uses only network switches rather than network hubs. The difference between network switches and hubs is that hubs forward data frames on all its ports at all times, while the network switches attempts to forward data frames only on those ports that are relevant for a certain data frame.
The Ethernet network may carry a protocol called Virtual Router Redundancy Protocol (VRRP), which is a known Internet Protocol. In other examples, the Ethernet network may carry Hot Standby Router Protocol (HSRP). The VRRP protocol provides automatic assignment of VRRP roles, routers in the Ethernet network. The VRRP roles include virtual router master and virtual router backup.
This kind of known Ethernet network may comprise multiple physical routers, which may be arranged and configured to represent a virtual router. The virtual router is assigned a VRRP MAC address. If the physical router that is routing data frames on behalf of the virtual router fails—or a link connected to a port of the physical router fails—, another physical router is selected automatically to replace it. A physical router that is receiving data frames addressed to the VRRP MAC address, e.g. at an interface of that physical router, has the role of virtual router master.
The Ethernet network also comprises a number of network switches for forwarding data frames within the Ethernet network towards their respective destinations, such as the virtual router, i.e. a physical router having the role of virtual router master. The data frames are Media Access Control (MAC) data frames, which include a destination MAC address field and a source MAC address field. See e.g. Institute of Electrical and Electronics Engineers (IEEE) 802.3 specifications.
When a network switch receives a data frame, it looks up a destination MAC address, located in the destination MAC address field of the received data frame, in a so called MAC address table in order to find out at which port of the network switch the received data frame should be feed. Additionally, the networks switch checks the source address field of the received data frame and notes at which port the received data frame was received.
When the MAC address in the source address field of the received data frame is not present in the MAC address table of the switch, the source address field and the port, at which the data frame was received, is put into the MAC address table for future use. It may also be the case that the MAC address in the source address field is mapped, according to the MAC address table, to another port. Then, the MAC address table needs to be updated. This process of populating the MAC address table is sometimes referred to as MAC learning.
Moreover, when the destination MAC address is not found in the MAC address table, the network switch forwards the received data frame to all its ports, except for the port at which the data frame was received. This creates additional traffic, due to data frames being forwarded to additional ports of the network switch, in the Ethernet network. The additional ports do not include the port at which the data frame was received. When the data frames are received at a router, and the router cannot find its own MAC address in the destination MAC address field of the data frames, the router is said to be flooded. Accordingly, the network switch is flooding data frames.
In an example, a host device, such as a Personal Computer or the like, sends data frames carrying payload data to the virtual router using the VRRP MAC address in the destination MAC address field. The virtual router is represented by a physical router that takes the role of virtual router master for a session identified by the VRRP MAC address. The session is typically a so called VRRP session which is known from VRRP terminology. When the physical router sends data frames carrying payload data back to the host, these data frames include a MAC address of the physical router in the source MAC address field. The physical router also sends special protocol frames, such as an announcement message according to VRRP i.e. an advertisement type message. These special protocol frames include the VRRP MAC address in the source MAC address field. Typically, the announcement message is sent periodically, e.g. through broadcast or multicast.
A failure, such as that a network switch goes down, may happen in the Ethernet network. A further switch, connected to the host device, will then no longer be able to forward data frames, intended for the master router, at the port connected to the network switch that went down. Thus, flooding will occur towards any other hosts or network switches, which are connected to the further switch mentioned above. See also FIG. 1b below.
A problem is that flooding occurs for a long time in the Ethernet network. The flooding will proceed until the announcement message is periodically sent. In order to shorten the time during which flooding occurs, the periodicity of the announcement message may be shortened. However, a shortened periodicity implies an increase of announcement messages, which take up valuable network resources which otherwise could be used for data frame carrying payload data.